This disclosure generally relates to the field of wind tunnel testing of aircraft designs. More specifically, this disclosure relates to systems and methods for evaluating and/or removing interference-induced distortion effects when analyzing an article under test, e.g., aircraft, aircraft part, or scale model of an aircraft or part, used to experimentally measure aerodynamic flow characteristics or signatures.
Fluid tunnels are used in a variety of applications to measure the effects of fluid flow over an object. For example, in aerodynamic testing, wind tunnels are used to measure the response of a test article, such as a scale model aircraft, to air passing over the aircraft. The wind tunnel provides a means to evaluate the test article in a controlled environment under conditions that are dynamically similar to conditions to which a full-size version of the aircraft may be subjected in actual flight. Air in the wind tunnel flows over the scale model aircraft at a controlled speed in order to evaluate the aerodynamic response of the scale model aircraft at different positions and attitudes. The attitude of the scale model aircraft may be defined in terms of roll, pitch, and yaw about the respective longitudinal, lateral, and vertical axes of the aircraft.
Scale model aircraft in a wind tunnel may be supported by a variety of different means including, but not limited to, a sting and balance mechanism extending out of an aft end of the aircraft. The sting forms part of a support mechanism for the aircraft and provides the ability to statically position or dynamically move the aircraft. For example, in changing the pitch angle of the aircraft, the sting may rotate the aircraft from a current attitude (e.g., a zero-pitch attitude or horizontal attitude) to a positive pitch attitude wherein the forward end of the aircraft is pointing upwardly relative to horizontal. In the positive pitch attitude, the wings of the aircraft may be oriented at a positive angle of attack relative to the direction of fluid flow through the wind tunnel. The direction of fluid flow may be assumed to be generally parallel to the tunnel walls or to the tunnel centerline, which may also be assumed to be horizontal.
The science of experimental measurement of off-body pressure signals emanating from supersonic wind tunnel models has existed since the 1940s. In the 1960s the fundamental theories of low sonic boom design were developed and experimentally validated. To perform the experimental validation of designs, it became common practice to obtain readings of the pressure signal at 100 or more locations with a static pressure probe. Typically the signal was painstakingly measured at one location at a time, by either traversing the probe thru the signal produced by a model at a stationary location, or by traversing the model over a stationary probe. In more recent times it has been found to be more cost effective and potentially more accurate to use techniques and hardware that can obtain readings at 100 to 500 locations simultaneously. An approach to enable the measurement of hundreds of off-body pressure signals simultaneously is to manufacture a pipe, plate, or rail (connected segments or one piece), with rows of tapped static pressure ports manufactured into the surface.
In the case of a pressure rail, if the rail has a finite width at the top and is placed a finite height from a tunnel wall, the pressure signals seen by a pressure rail used for sonic boom measurement may be significantly distorted by a number of fluidic phenomena. The phenomena include, but are not limited to, boundary layer dissipation, signals reflected off the rail surfaces, and signals reflected off the wind tunnel walls. Depending on the characteristics of the facility, the test conditions, the signal characteristics generated by a specific model, and the geometry of the rail, the rail will experience different levels of distortion relative to what the model would produce under the most ideal conditions. The existing solutions are to accept the distortion, develop distortion limiting measurement hardware, or not use a pressure rail at all. Using an alternate technique, like a pressure probe and translating model, increases the time to obtain a pressure signature from seconds to 30 minutes or more. Low-interference rails use up large portions of a wind tunnel test section. This translates to a requirement to only test in the largest and most expensive wind tunnels, driving up data acquisition cost.
For some combinations of rail geometry, model geometry, wind tunnel facility, and test conditions, very low levels of distortion are achievable. In situations where the application of a low-interference approach is uneconomical or impractical, due to model geometry, facility characteristics, or test condition requirements, an effective method of correcting for pressure probe and tunnel wall interferences would be highly advantageous.
Accordingly, there is a need for a method to correct measured off-body pressure signals for major interferences during wind tunnel testing, so that a higher-quality pressure signature can be calculated from the measured data.